Transport of leased-line services has traditionally occurred over optical networks utilizing the Synchronous Optical Network (SONET) protocol. This is because a fundamental characteristic of a SONET network is synchronous operation, e.g. the availability of a common synchronization or frequency source at SONET network elements. SONET networks rely on the distribution of synchronization from a common clocking source.
The source of common clocking is a highly accurate clock known as a Stratum clock. The different levels of the Stratum hierarchy and the characteristics of each are described in the Telcordia GR-1244-CORE specification, incorporated herein by reference in its entirety. A commonly used external synchronization source is a Stratum 1 global positioning system (GPS) clocking source. For a SONET optical metropolitan access network, the external synchronization source may be attached directly to a device within the network known as the synchronization source device. If there is no external synchronization source directly attached, the synchronization source device may also derive synchronization from any of its external leased line interfaces, as configured by the network operator. Each of the other devices in the network is configured to accept synchronization from one of its high-speed SONET interfaces. An example is shown in FIG. 1 for a metropolitan access bi-directional ring. In FIG. 1A, the paths over which synchronization is distributed originate from a synchronization source device 12 that obtains synchronization from one of its external interfaces. All other devices in the SONET ring derive synchronization from the synchronous frame structure of the incoming data on one of its high-speed SONET interfaces. Devices external to the SONET ring such as device 13 (external to the SONET ring) connected to node 10 and the device 14 (external to the SONET ring) connected to node 12 can derive timing from the nodes within the SONET network.
SONET networks, unfortunately, are not the best solution for the transport of packetized data in metropolitan access networks. Connectionless, asynchronous, packet-switched networks are the most cost-efficient and capacity-efficient solutions for this transport. However, these networks have never required the distribution of synchronization for their operation. It is only now that synchronization distribution within asynchronous networks has become important, as service providers strive to implement a network infrastructure optimized for the transport of packetized data but also capable of the transport of leased-line services. A mechanism to perform this function is critical to reduce the infrastructure cost of metropolitan access networks.
In this specification, the term “synchronization” will be used in phrases such as “synchronization source” interchangeably with the terms “timing” or “clocking”.
The mechanism described in this specification utilizes novel functions and capabilities on the media access control (MAC) layer. Some general capabilities required for network synchronization, such as prioritized insertion of control symbols on the MAC layer, enable other novel functions to be performed on the MAC layer that are unrelated to network synchronization.